Gypsum Surface Research Articles

Selective adsorption behaviors and mechanisms of benzyl quaternary ammonium salt on quartz and gypsum: A combined experimental and theoretical study

Benzyl quaternary ammonium salt is an effective collector of quartz from gypsum. To investigate the selective collection mechanism of tetradecyl dimethyl benzyl ammonium chloride (TDBAC) for quartz from a microscopic view, the adsorption experiments, first-principles calculations based on density functional theory (DFT), and molecular dynamics (MD) simulation were performed in this work. The adsorption experiment results show that the adsorption capacity of TDBAC on the quartz surface was 1.6 × 10−6 mol/m2, much larger than that on the gypsum surface (1.0 × 10−7 mol/m2). The interaction energy of TDBAC with quartz surface was −20202.98 kJ/mol, much smaller than that with gypsum surface (-78.37 kJ/mol). DFT calculation results indicate that the quartz surface was more likely to interact with TDBAC through electrostatic attraction since the average negative charge and exposure oxygen density of the quartz surface were both greater than those of the gypsum surface. After the adsorption of TDBAC, there were hydrogen bonds formed between the hydrogen atoms in TDBAC and the oxygen atoms on the quartz surface. This work confirmed that the highly selective adsorption of TDBAC on quartz surface was attributed to the strong electrostatic attraction and hydrogen-bond interaction.

Desilication and purification of phosphogypsum waste by flotation using a photosensitive collector

Phosphogypsum (PG) is the main by-product of wet phosphoric acid production and has seriously polluted the environment. In this study, PG was purified by reverse flotation using a photosensitive azobenzene surfactant (PASF). After reverse flotation, the content of gypsum in PG increased from 89.16 % to 93.32 %, the whiteness of PG increased from 25.62 % to 60.9 %, and the content of quartz decreased from 4.87 % to 2.12 %. The Zeta potential of quartz and gypsum became positive after being treated with PASF, indicating that PASF enhanced the floatability of quartz and gypsum. The results of FTIR and XPS indicated that there were both electrostatic attraction and hydrogen bonding between PASF and quartz. Density functional theory (DFT) calculations indicated that the adsorption of PASF on the quartz surface was more stable than on the gypsum surface. Upon exposure to UV irradiation, the molecular structure of PASF was transformed from trans to cis. Meanwhile, the adsorption amount of PASF on the quartz surface was decreased and the rigidity of the corresponding adsorbed layer was increased. This work provides a method to purify PG waste, which is important for the comprehensive utilization of PG.

Scale inhibition mechanism of calcium sulfate by gum Arabic: Experimental testing, quantum chemical calculations, and molecular dynamics simulations

In this paper, the scale inhibition mechanism of gum arabic (GA) on calcium sulfate fouling is theoretically and experimentally investigated. A combination of experiments, quantum chemical calculations, and molecular dynamics simulations are carried out. The results demonstrated that GA has the potential to inhibit the precipitation process of calcium sulfate fouling significantly. As the concentration of GA increases, the scale inhibition efficiency also increases. Moreover, with a concentration of 6.8 g/L of calcium sulfate in the solution, it is shown that the scale inhibition efficiency of 1 g/L GA is 97.5 %. Additionally, it is noted that the variation in the solution pH does not significantly affect the GA scale inhibition effect. It was highlighted that carboxyl groups, hydroxyl groups, and ether bond oxygens in the GA molecule are the functional groups that exhibit scale inhibition. These groups serve as the low electronegativity regions with strong interactions with calcium ions. In addition, GA can be adsorbed on the active growth sites on the gypsum surfaces to form a dense, self-assembled barrier film. This film aids in inhibiting the calcium sulfate precipitation process. The findings of this study rank the scale inhibition of GA at different crystal surfaces as follows: gypsum (010) > gypsum (-111) > gypsum (120).

Profiling Novel, Multifunctional Silane-Phosphonate Consolidants for the Mitigation of Gypsum Stone Deterioration via Concerted Autocondensation/Surface Complexation Processes.

Mineral gypsum (selenite) stones have been used extensively by ancient Cretans in the Minoan Palace of Knossos (Crete, Greece), mostly for building and ornamental purposes. Exposure of mineral gypsum to environmental stresses (temperature fluctuations, rain, air-borne pollutants, soluble salts, etc.) causes solubility-driven degradation, and loss of cohesion of the crystal aggregates, with ensuing aesthetic degradation. In this work, the efficiency of four consolidants for artificial gypsum specimens is presented and evaluated based on drilling resistance measurements [drilling resistance measuring system (DRMS)]. Two of them (commercial names RC-70 and RC-90, RC = Rhodorsil Consolidante) are alkoxysilane-based and they are considered as benchmark consolidants. The other two [3-(trihydroxysilyl)propyl methylphosphonate monosodium salt, TRIMEPHONA, and 3-(trihydroxysilyl)propylamino-diphosphonate, TRIPADIPHOS] are multifunctional consolidants because they possess a self-condensable (after hydrolysis) trihydroxysilyl [-Si(OH)3] moiety and phosphonate groups (one in the former, two in the latter). Consolidants RC-70 and RC-90 exhibit rather low consolidation effectiveness. This is not unexpected, as these are alkoxysilane-based and act simply as "fillers" for the pores of the gypsum. Consolidant TRIMEPHONA demonstrates an enhanced level of consolidation action. This is due to its double functionality, i.e., the presence of an anionic phosphorus-based moiety that anchors onto the gypsum surface, and a condensable silane triol [-Si(OH)3] unit. Consolidant TRIPADIPHOS shows excellent gypsum consolidation features and is much more efficient (per unit concentration) than all other tested consolidants. This is assigned to its better gypsum anchoring ability via surface Ca-complexation. Selected compressive strength studies were performed on gypsum samples treated with the phosphorus-based consolidants, and corroborate the findings from DRMS. To shed further light on possible binding modes of the phosphonate moiety on surface Ca2+ sites in gypsum, two model compounds were synthesized and structurally characterized, Ca-C2D and Ca-C3D (C2D = ethylamino-di(methylenephosphonic acid) and C3D = propylamino-di(methylenephosphonic acid).

Origins of authigenic gypsums and carbonate minerals in sediments at a cold seep site in the Sea of Marmara

In the Western High of the Sea of Marmara, where pervasive gas hydrate and hydrocarbon gas seepage occurs, late Pleistocene-Holocene sediments are composed of a lower lacustrine and an upper marine unit. The sedimentary evolution, gypsum formation and carbonate anomaly, under the complex sedimentary environment at cold seep sites on the Western High, have not been systematically well explained. The transition from the lacustrine to marine unit occurs at around 7.5 m below the sea floor (mbsf), with a sulfidization front present in the lacustrine sediments just below it, and the marine unit includes a sapropel layer between 5.20 and 7.20 mbsf. The sedimentary sequence is characterized by authigenic minerals including pyrite, carbonates, and gypsum in the upper 0.50 mbsf, corresponding to the present-day sulfate-methane transition zone (SMTZ) near the seafloor. Mineralogical, chemical and isotopic compositions (13C, 18O, 34S, 26Mg) of the sediments in the core studied show their close relationship with intensive sulfate-driven anaerobic oxidation of methane (AOM) and other hydrocarbons. The dissolution of carbonate, related to the gas and oil migration, and organic matter enrichment might be the main cause of carbonate deficiency in the sapropel unit. Heavy carbon isotopic composition (δ13CCarb = 6.98‰) of carbonate at 4.30 mbsf implies that the carbon might be the residual by-product of subsurface biodegradation of seeping petroleum. The formation of secondary gypsum and partial dissolution of carbonate in the Sea of Marmara is commonly considered of being associated with the aerobic oxidation of pyrite. However, the euhedral authigenic gypsum studied might be with different origins, possibly including anaerobic oxidation of Fe-sulfides, carbonate deficiency in the sapropel unit and AOM. Gypsums below SMTZ are with the size of hundreds of microns. Linear pits and grooves are developed on these gypsum surfaces, probably due to corrosion by the methane-rich fluid of latest hydrocarbon seepage. Platy hexagonal gypsum in SMTZ, without any corrosion on crystal surface, might be associated with the changing flux of the acidic composition (e.g. CO2) in the seepage. Depleted-26Mg by about 1‰ in the SMTZ might be caused by the upward seepage of hydrocarbons. Magnesium isotope of authigenic carbonate could be a valid proxy for the recognition of SMTZ in a hydrate geo-system.

Selective flotation separation of gypsum and quartz using dodecyl amine hydrochloride as collector: Mechanism and application

The selective flotation separation of gypsum and quartz in phosphogypsum (PG) is an urgent problem that is very important for the high‐value utilization of PG. In the work, dodecyl amine hydrochloride (DH) was introduced as a collector for the selective flotation separation of gypsum and quartz. The flotation property and selective mechanism of DH on the surface of gypsum and quartz were researched by single mineral flotation experiments, Fourier transform attenuated total reflection infrared spectroscopy analyzer, zeta potential analyzer, X‐ray photoelectron spectrum, molecular dynamic (MD) simulation, etc. Additionally, H+ was introduced into the mineral‐water system simulated by MD to take into account the effect of acidic conditions on the adsorption of DH. The pre‐adsorption of H+ on the quartz surface under strongly acidic conditions hindered the electrostatic force adsorption of DH on the quartz surface. Furthermore, DH was adopted as a collector in the direct flotation recovery of gypsum from PG, and the gypsum concentrates with productivity of 69.85%, CaSO4·2H2O content of 96.33%, and whiteness of 55.00% were obtained. The SiO2 content in the gypsum concentrate decreased from 9.08% to 0.616%. It suggested that DH could serve as a promising collector in the selective separation of gypsum and quartz in PG via flotation.

Effect of propolis on the mechanophysical properties of addition silicon dental impression material

Background Dental impressions should be disinfected to stop the transmission of germs among dental workers since they provide a risk of diseases. The aim of this study was to examine the effects of two disinfectants (16 mg/ml propolis and 5.25% sodium hypochlorite) on the detail reproduction, linear dimensional change, compatibility with gypsum products, wettability, and surface roughness of addition silicon material. Methods Ninety specimens were created using Addition silicon impression material, and they were then divided into three groups at random. Ten specimens from each test group were used in each test. Utilizing apparatus made in accordance with ISO 4823:2015, detail reproduction, linear dimensional change, and compatibility with gypsum products are assessed. A digital Profilometer was uutilized to evaluate surface roughness, and a Goniometer was utilized to evaluate wettability by taking the contact angle. Before testing, the specimens were immersed for 10 minutes each in two disinfection solutions: 16 mg/ml propolis and 5.25% NaOCL. The control group received no disinfection. One-way ANOVA tests and the Kruskal-Wallis test, which was statistically significant at a level of p 0.05, were used to examine the data. Results Comparing the linear dimensional change, surface roughness, and wettability of Addition silicon specimens immersed in 16 mg/ml propolis, 5.25% NaOCL, or the control group demonstrated statistically significant differences (p< 0.05), while details reproduction and compatibility with gypsum revealed statistically no significant differences (p > 0.05). Conclusion Within the parameters of this study, the addition silicon can be immersed in 16 mg/ml propolis for 10 min. to disinfect it without compromising its dimensional accuracy, detail reproduction, compatibility with gypsum products, surface roughness, or wettability.

New Hydrophobic Treatment for the Protection of Gypsum Artifacts

Gypsum has been widely used in traditional and monumental architecture in many countries of the Mediterranean area; however, due to its low water resistance and low mechanical strength, it can be subject to physical, chemical and biological degradation. To improve the preservation of gypsum-based plaster artifacts, a new hybrid inorganic-organic hydrophobic treatment is proposed in this work. This treatment is based on nanometric magnesium hydroxide (Mg(OH)₂) and organically modified silica (OrMoSil), a class of materials with intermediate performances between ceramic materials and polymers. The behaviour of the product was verified both as a protective on gypsum surface, as well as a consolidant added to the mixture of gypsum-based plasters. The treatment does not alter the aesthetic appearance of the surface, as verified through colorimetric analysis. As demonstrated by capillary water absorption and static contact angle measurements, the new hydrophobic protective coating proved to be very effective. Moreover, the new treatment is effective as a consolidant, leading to an improvement of the mechanical properties and to a reduction of the depth of carbonation. The new hydrophobic nanocomposite product is therefore a promising material for the protection and consolidation of gypsum monuments and artifacts of cultural interest.

Study on the occurrence state of main components of phosphogypsum dihydrate and its impurity distribution.

The dihydrate phosphoric acid process is the mainstream technique. However, the phosphogypsum (PG) produced contains high levels of impurities such as phosphorus and fluorine, severely constraining its valorization. In order to elucidate the occurrence patterns of phosphorus and fluorine impurities in PG, this study employed analytical methods including XRF, XRD, AMICS (Automated Mineralogy Integrated with Chemistry System), XPS, and chemical element balance analysis. We investigated the occurrence states of phosphorus, fluorine, silicon, iron, and aluminum elements in PG from wet-process phosphoric acid production, as well as the distribution characteristics of phosphorus and fluorine impurities. Additionally, we utilized Density Functional Theory (DFT) calculations to determine the binding energies of major minerals with water-soluble phosphate and fluoride groups, and analyzed the zeta potentials of gypsum and quartz mineral surfaces. The results indicate that the main mineral phases in PG are gypsum, quartz, potassium silicate minerals, aluminosilicate minerals, and hematite, predominantly occurring in monomineralic forms. Phosphorus impurities primarily exist in calcium silicate and hematite minerals, while fluorine is mainly associated with gypsum and potassium silicate minerals. DFT calculations demonstrate strong binding energies between calcium silicate and hematite minerals with PO4 3-, as well as between gypsum and quartz minerals with F-. The acidic conditions in the separation tank during wet-process phosphoric acid production may contribute to the distinctive distribution characteristics of phosphorus and fluorine impurities in PG.

Enhancement and mechanism of macro-defect free (MDF) gypsum water resistance achieved by hydrophobic modification

Macro-defect free (MDF) gypsum is characterized by high strength and has the potential designed to advanced functionalization materials. However, the application of MDF gypsum is still limited by its poor water resistance. Here, we propose to modify the gypsum surface to hydrophobicity at molecule scale by adding potassium methylsilicate. The compressive strength, softening coefficients, and water absorption were subjected to rigorous testing, while elucidating the underlying mechanisms of water resistance enhancement through micro-structural analysis and organic-inorganic interactions. The results show that when adding 0.25% potassium methylsilicate, the wet compressive strength, softening coefficients are increased by 30.1%, 28%, respectively. And potassium methylsilicate can change the gypsum surface to hydrophobicity, which delaying the initial rapid water absorption stage and decrease both the 2 h and 24 h water absorption. The addition of potassium methylsilicate leads to a reduction in the dried compressive strength, attributed to the decrease in intrinsic dihydrate crystal strength. Upon the introduction of potassium methylsilicate, there is a transformation in gypsum crystal morphology from strip-like to lamellar structures. This alteration is accompanied by the formation of a potassium methylsilicate film coats the surface of gypsum, impeding water-gypsum contact. The potassium methylsilicate film coats on the surface of gypsum, which hinders the contact of water and gypsum.

Assessment of Materials on Covid-19 Spread Prevention in Public Buildings

This research is an attempt to suppress the spread of COVID-19 in public buildings through the application of materials that may prevent the spreading of SARS-CoV-2 while at the same time improving audial comfort in such buildings. Study on the capability of materials to break further spreading of the virus applied by measuring the presence of SARS-CoV-2 RNA on the material based on exposure time (0, 3, 6, and 9 minutes). This RNA tracing procedure is repeated on the surface of aluminum, tiles, carbon steel, glass, gypsum, copper, and painted cement on the wall. The results showed that the CT value (cycle threshold) of all materials was approximately 25, which means that they still have the potential to spread the virus. Slightly different results occur on the gypsum surface where the viral envelope (gene E) is not detected with a CT value of 36, meaning no chance of the virus spreading. The same materials will undergo measurement of sound absorptive characteristics to assess their effect on the audial comfort in the building. The results of the study are expected to be a reference before their application as interior materials in public buildings.

Dryland Endolithic Chroococcidiopsis andTemperate Fresh Water Synechocystis Have Distinct Membrane Lipid andPhotosynthesis Acclimation Strategies upon Desiccation andTemperature Increase.

An effect of climate change is the expansion of drylands in temperate regions, predicted to affect microbial biodiversity. Since photosynthetic organisms are at the base of ecosystem's trophic networks, we compared an endolithic desiccation-tolerant Chroococcidiopsis cyanobacteria isolated from gypsum rocks in the Atacama Desert with a freshwater desiccation-sensitive Synechocystis. We sought whether some acclimation traits in response to desiccation and temperature variations were shared, to evaluate the potential of temperate species to possibly become resilient to future arid conditions. When temperature varies, Synechocystis tunes the acyl composition of its lipids, via a homeoviscous acclimation mechanism known to adjust membrane fluidity, whereas no such change occurs in Chroococcidiopsis. Vice versa, a combined study of photosynthesis and pigment content shows that Chroococcidiopsis remodels its photosynthesis components and keeps an optimal photosynthetic capacity at all temperatures, whereas Synechocystis is unable to such adjustment. Upon desiccation on a gypsum surface, Synechocystis is rapidly unable to revive, whereas Chroococcidiopsis is capable to recover after three weeks. Using X-ray diffraction, we found no evidence that Chroococcidiopsis could use water extracted from gypsum crystals in such conditions as a surrogate for missing water. The sulfolipid sulfoquinovosyldiacylglycerol becomes the prominent membrane lipid in both dehydrated cyanobacteria, highlighting an overlooked function for this lipid. Chroococcidiopsis keeps a minimal level of monogalactosyldiacylglycerol, which may be essential for the recovery process. Results support that two independent adaptation strategies have evolved in these species to cope with temperature and desiccation increase and suggest some possible scenarios for microbial biodiversity change triggered by climate change.

Influence of H3PO4 and H2PO4− on the performance of PCE in hemihydrate gypsum pastes

Phosphorus impurities are the main factor causing the performance deterioration of β-hemihydrate phosphogypsum (β-HPG), and polycarboxylate superplasticizer (PCE) is a common admixture to improve β-HPG properties. Revealing the interaction mechanism between phosphorus impurities and PCE is significant for efficiently utilizing β-HPG. This study systematically investigated the influence of H3PO4 and H2PO4− on the performance of PCE in hemihydrate gypsum pastes. The results indicate that H3PO4 and H2PO4− weakened the dispersion ability of PCE, prolonged the setting time of hemihydrate gypsum pastes, and deteriorated the mechanical properties. H3PO4 had the most significant adverse effect on the performance of PCE and the mechanical properties of hemihydrate gypsum, while H2PO4− had the most noticeable impact on the setting time. The conductivity, adsorption amount, and X-Ray photoelectron spectroscopy results showed that H3PO4 and H2PO4− formed an adsorption layer on the surface of hemihydrate gypsum through the interaction with Ca2+. Then, the adsorption layer interfered with the adsorption behavior of PCE, resulting in a decrease in its adsorption amount, which was responsible for the weak dispersion ability. In addition, the hydration process of hemihydrate gypsum was hindered due to the formation of the adsorption layer, leading to prolonged setting time and a low degree of hydration, contributing to the deterioration of mechanical properties.

Investigations into the rheology and early-age properties of gypsum-based materials with different sticky rice contents

To use gypsum-based materials (GM) in a green manner, more natural starch derivatives are being developed as GM modifiers. In this paper, the effects of different sticky rice contents on the rheology and early-age properties of modified GM pastes were investigated. The results showed that the amylopectin in sticky rice was adsorbed on the hemihydrate gypsum surface, increasing the steric hindrance and reducing the particle size and spacing. In all dosage ranges (0.00 wt.%∼3.33 wt.%), relative to the control group, sticky rice increased the yield stress, maximum shear stress, and 28-day compressive strength of GM pastes by 2302.0%, 366.5%, and 134.9%, respectively, and decreased the fluidity by 64.5%. When the critical content exceeded 2.67 wt.%, the thixotropy, hydration heat, and compressive strength values of GM pastes were inhibited, decreasing by 25.4%, 24.8%, and 18.0%, respectively; the setting time was prolonged by 35.6%. This phenomenon was caused by the decrease in the Ca2+ complexation rate due to the overadsorption of amylopectin that led to the wrapping of hydration products. According to the above research, a model based on homogenization assumptions was speculated to describe the solidification mechanisms of GM pastes under sticky rice.

Agar Foam: Properties and Cleaning Effectiveness on Gypsum Surfaces

In the past decade, the usage of soft materials, like gels, has allowed for a better control of the water release process into the substrate for cleaning interventions. Agar—a natural polysaccharide harvested from algae—has been used to perform cleaning of stone materials, gypsum works, and paintings with remarkably positive results. Agar presents the great advantage of being cheap, easily available, fast to produce and not toxic, allowing for more sustainable conservation works. More recently, a new type of agar fluid, agar foam, promises further control of the water release and ease of application on delicate surfaces. In the present study, this new type of agar, CO2 and N2O foams, has been characterized and compared with the conventional sol/gel agar system. Moreover, the cleaning effectiveness of the agar foam was tested both in laboratory conditions and in two case studies: a historical gypsum from the porch framing of the Abbey of Nonantola, and the 20th century gypsum cast of the Pietà Rondanini by Michelangelo, located in the Sforza Castle in Milan. The obtained results show that foaming changes the sol-gel transition temperature of the agar gel as well as incrementing its dissipative behavior. When freshly applied, the foams flow with a reduced velocity, thus allowing a better control and ease of application. Once gelified, they act as a soft solid-like material, as shown by their rheological properties. Moreover, it was found that CO2 foam slightly reduces the water release to the surface, while maintaining the moldability and ease of application. The study allows for the conclusion that agar foam offers an interesting alternative for delicate surfaces, with a non-coherent mineral deposit, and with complex geometries that often represent a challenge for the conventional agar applications

Conversion of phosphogypsum into α-hemihydrate in the presence of potassium acid phthalate and Ca2+: Experimental and DFT studies

The preparation of α-hemihydrate is a practicable approach for resource utilization of phosphogypsum (PG) waste. Potassium acid phthalate (KAP) was used to control the crystal morphology of α-hemihydrate, the inhibition behavior and mechanism of KAP in the absence and presence of Ca2+ were clarified by experimental and density functional theory (DFT) studies. The results show that KAP can effectively regulate the morphology of α-hemihydrate from acicular whisker to short columnar crystal. Interestingly, the KAP has little effect on the conversion rate of PG in pure NaCl solution, but it could obviously inhibit the conversion of PG into α-hemihydrate by prolonging the induction period in the mixed solutions of NaCl and CaCl2. X-ray photoelectron spectroscopy (XPS) analysis and DFT calculation showed that KAP molecule cannot adsorb on the dihydrate gypsum surface in the absence of Ca2+. Chemical adsorption was formed between O of KAP, Ca2+ and O of dihydrate gypsum (0 2 0) surface (OKAP- Ca2+-Osurf) in the presence of Ca2+, thus KAP can be stably adsorbed on the dihydrate gypsum surface and prevent its dissolution, resulting in the conversion of PG into α-hemihydrate was inhibited.

Insights into the Role of Na+ on the Transformation of Gypsum into α-Hemihydrate Whiskers in Alcohol-Water Systems.

Alcohol–water solution-mediated transformation of gypsum into α-hemihydrate (α-HH) whiskers provides a green alternative for the high-value-added recycling of flue gas desulfurization (FGD) gypsum. However, the role of non-lattice cations during the transformation is still unclear. We report an evolution from “boosting–retarding” to “boosting–retarding–boosting” and finally to “boosting only” effect of non-lattice Na+ functioned by the concentration of ethylene glycol (EG) in water solutions. The driving force increased almost linearly upon the introduction of Na+ through the formation of ion pairs, and a higher slope was obtained at a higher EG concentration. Adsorption of Na+ ions and solidification of eugsterite on gypsum surfaces blocked the nucleation sites of α-HH. The retarding effect first rapidly increased and gradually approached a limit, following a parabolic trend after Na+ ions were introduced. Pentasalt, with a structure similar to that of α-HH, precipitated on the gypsum surface at higher c(Na+). The interaction of the driving force and the structural evolution of calcium sulfate ionic clusters accounts for the evolution of transformation kinetics. The retardation zone was compressed with the increase in EG volume ratios, and a monotonic boosting effect upon Na+ was observed at a 35.0 vol % of EG. Nucleation kinetics dominates the aspect ratio of α-HH whiskers. This study may provide a significant guidance for the utilization of FGD gypsum.

Surfactant Adsorption to Gypsum Surfaces and the Effects on Solubility in Aqueous Solutions.

Vibrational sum frequency generation (VSFG) spectroscopy, conductometric titration measurements, and EDX elemental mapping were used to examine surfactant adsorption to the gypsum (010) surface and assess the effects of surfactant adsorption on gypsum solubility in aqueous solutions. Sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium chloride (DTAC) were used as anionic and cationic surfactants, respectively. Gypsum/SDS interactions result in an ordered precipitate layer on the gypsum surface after water evaporation; gypsum/DTAC interaction did not show a similar effect, despite exposure of gypsum to equivalent amounts of surfactant. VSFG spectra showed that SDS molecules adsorb with their chains parallel to the gypsum surface; spectra from gypsum surfaces treated with DTAC, however, showed no measurable response, implying that these surfactants form disorganized aggregates with no polar ordering. Vibrational data were supported by independent EDX measurements that show a uniform distribution of SDS across the gypsum surface. In contrast, element-specific EDX images showed that DTAC clustered in tightly localized patches that left most of the gypsum surface exposed. The uniform adsorption of SDS on the gypsum surface suppresses long-term dissolution up to 40% when compared to samples exposed to DTAC. Gypsum samples in DTAC-containing solutions lose approximately the same amount of material to dissolution as samples immersed in pure water.

Characterization of Wall Paintings of the Harem Court in the Alhambra Monumental Ensemble: Advantages and Limitations of In Situ Analysis.

Non-invasive techniques (X-ray fluorescence, XRF, and Raman spectroscopy) were used for the study of the Hispano Muslim wall paintings. Principal component analysis (PCA) was performed on the semi-quantitative XRF results directly provided by the in-built factory calibrations with minimum user manipulation. The results obtained were satisfactory and highlighted differences and similarities among the measurement points. In this way, it was possible to differentiate the decorations carried out on gypsum plasterwork and the wall paintings over lime plaster. The color palette, revealed by combining the results from XRF and Raman spectroscopies, comprised the pigments hematite, lapis lazuli, cinnabar (in poor conservation state), and possibly, carbon. Evidence of past interventions was also provided by PCA on XRF data, which detected the presence of Pb, Ba, and Zn in some areas. Furthermore, the preparation layers have been studied in detail on cross-sections of two microsamples. Several layers of lime plaster with a compact microstructure have been observed. The characteristic of the pictorial layer and the identification of calcium oxalate point to the use of a secco-technique. The main alteration identified was a gypsum surface layer covering the painting and signs of plaster deterioration due to gypsum migration to more internal areas. Finally, the comparison with the observations made by restorers in previous interventions on these paintings revealed the importance of the representativeness gained with the in situ study, which enabled the analysis of a high number of areas.

Metal (Fe, Cu, and As) transformation and association within secondary minerals in neutralized acid mine drainage characterized using X-ray absorption spectroscopy

The formation and transformation of secondary minerals in areas affected by neutralized acid mine drainage (AMD) determine the behavior of toxic elements. To better understand the binding form of toxic elements in the secondary minerals, we focused on the distribution of secondary precipitates and heavy metals in neutralized AMD near a typical Cu-polymetallic deposit located in central Tibet. Compared to background values, the average heavy metal (As, Cu, Co, Cu, Mo, Pb, and Zn) content is higher in sediments close to the neutralized AMD drain. X-ray absorption spectroscopy (XAS) characterization shows the Fe in the suspended particulate matter (SPM) first aggregate as small Fe(III) octahedra clusters and then precipitate as ferrihydrite along the river. The XAS results also show the by-products of neutralization—ferrihydrite and sulfate—are the primary forms of Fe and S in the sediments, respectively. Gypsum retains Cu in the sediments via structural fixation, surface adsorption, and surface coprecipitation. The difference of Cu-O coordination number between the SPM and the sediments reveals the Cu sulfate coprecipitation on the surface of gypsum accompanied by SPM transporting along the river. XAS results confirm that ferrihydrite act as an essential sink for heavy metals, especially As. As(III) mineral in the sediment near the neutralized AMD outfall is dissolved and oxidized to arsenate downstream, which then adsorb to ferrihydrite and/or form As(V)-Fe minerals along the river. Gypsum and ferrihydrite have the potential to release heavy metals with changes in water chemistry, which may pose a threat to the safety of drinking water for downstream residents. The results deepen the understanding of the influence of secondary minerals (especially gypsum and ferrihydrite) on the migration and transformation process of heavy metal pollutants in mine drainage system.